The instant invention relates to laser technology and more particularly, to a device for selecting a single output mode from a laser beam.
The laser is widely recognized as a source of coherent light or energy which operates at a very specific wavelength. However, lasers, such as ion lasers, actually may operate over a range of output wavelengths, referred to as the laser gain curve, centered on that specific nominal wavelength. For example, a helium-neon (HeNe) laser operating nominally at 6328 angstroms (A) actually may operate anywhere within a gain curve having a frequency spread of approximately 1.5 Ghz, or 0.02 A about the center point of the 6328 A nominal wavelength. Since the laser cavity is a type of Fabry-Perot interferometer, the energy output is not a continuum as suggested by the gain curve, but includes a number of frequency pass bands, termed axial modes, defined by the mirror spacing in the cavity. Specifically, the pass bands are separated by c/2 l, where c equals the speed of light and l equals the mirror spacing in the laser cavity. For example, for a one meter long cavity, the pass bands are separated by 150 Mhz. Accordingly, the actual laser output may be represented as a distribution of numerous discrete wavelengths, each separated from the next by such a pass band and the total number of output wavelengths distributed over the entire laser operating gain curve. For example, in an HeNe laser having an operating frequency spread of 1.5 Ghz and a one meter optical cavity, there are approximately 10 separate lines of output radiation separated by the 150 Mhz pass band spacing.
While, for many applications the use of laser output radiation having such a described frequency spectrum is satisfactory, a number of other applications, such as high resolution spectroscopy and holography, require much narrower frequency distributions. This need for narrowing the output frequency spectrum has become particularly acute with the development of the broad band, tunable dye laser. Such dye lasers are capable of operation over a relatively broad range of output wavelengths and require, for maximum usefulness, some means for reducing the bandwidth of the output, preferably to a single axial mode. It is also desirable that means be provided for selecting such a single axial mode at any point within the full frequency spectrum of the laser output. This function conventionally is performed by the use of an optical element known as a Fabry-Perot etalon. In the prior art, the types of etalon known include the solid type comprising a block of glass or other similar optical material having opposite faces accurately parallel to one another, and the air- or gas-spaced type. In either of these types of etalons, the structure includes two accurately parallel surfaces inclined very slightly to a normal to the optical path and through which the optical path passes. The etalon parallel surfaces are appropriately spaced to form a resonant cavity therebetween, such that certain frequencies of the beam energy are transmitted through the etalon, while others are internally reflected by the etalon surfaces off the laser cavity optical axis and thus out of the beam passing through the etalon.
When etalons are tilted, as is necessary to prevent them from coupling energy back within the laser cavity themselves, the inherent multiple internal reflections cause "walk-off" losses in the beam, as described in "Losses Introduced by Tilting Intra-Cavity Etalons" by Walter R. Leeb, Applied Physics 6, 267-72 (1975), and references cited therein. The well known thick, solid etalon, because of its uniform index of refraction and relatively great spacing between the reflective surfaces, has been found to have low walk-off losses but also possess no means for rapid variation of the spacing between the reflective surfaces as is necessary to tune the etalon. Conventional air or gas-spaced etalons generally comprise a pair of relatively thin plate-like glass members parallel to and aligned with one another and spaced apart from one another. In the air-spaced etalon the laser beam passes through both plates, the outwardly facing surfaces of those plates being provided with an anti-reflective coating and the mutually facing surfaces of the respective plates being provided with a partially reflective coating. By varying the spacing between the plate-like members of the etalon, the wavelength of the output radiation of the laser may be readily adjusted. However, with this conventional air-spaced etalon the relatively thin plate-like members and the interposition of the air or gas space between them results in relatively large walk-off losses, such losses often being greater than is permissible for use with certain types of lasers, such as dye lasers. An improvement to the conventional air-spaced etalon is disclosed in Cassels U.S. Pat. No. 3,775,699 in which the optical elements comprise a pair of spaced prisms with the outwardly facing surfaces of those prisms inclined at Brewster's angle to avoid the necessity of using an anti-reflection coating on those surfaces. In this tilted prism type etalon the laser beam passes through the gap between the two prisms in a direction nearly normal to the two mutually facing surfaces of the two prisms. While the etalon as disclosed in the Cassels patent is more efficient optically than the spaced parallel plate variety, it has several major deficiencies. One serious deficiency is the substantial offset occurring between the beam entering the etalon and the beam leaving it. Further, because of the dispersion of the different wavelengths of light in the prism material, as described in the Cassels patent, the offset between the input and output beams varies with the optical wavelength. This dispersion inherent in the prisms thus requires, for efficient operation at each desired wavelength, tilting of the entire etalon to a different angle with respect to the incoming light beam, thus making necessary repeated angular adjustment of such etalon when used over a broad range of wavelengths.